Four wheel steering has in the last few years gained some modicum of commercial success in passenger vehicles, and it appears that by the early 1990s four wheel steering systems will be utilized in most performance-type passenger vehicles and later in the decade in almost all passenger vehicles, and all this projected four wheel steering expansion is due to the improved handling characteristics of four wheel steering.
The fundamental advantage in four wheel steering is that for a given angular movement of the front wheels, a given vehicle speed, and a given distance traveled, the longitudinal axis of the vehicle will turn through a far greater angle with four wheel steering than with two wheel steering. This is because the rear wheels in a turn in four wheel steering tend to track for outside the turn path of the front wheels, swinging the rear end of the vehicle markedly outside during a turning maneuver.
For purposes of discussion further in this description, the central longitudinal axis of the vehicle is referred to as a reference axis, and the angles through which the front wheels are turned during any turning movement with respect to this reference axis are incrementally .theta..sub.F1, .theta..sub.F2 --.theta..sub.FN, and the incremental angles through which the rear wheels are turned with respect to the same reference axis are turned .theta..sub.R1, .theta..sub.R2 --.theta..sub.RN. Ideally, and for maximum steering response, .theta..sub.R will always equal .theta..sub.F, that is, the angular position of the rear wheels should always be equal to the angular position of the front wheels, on the opposite side of the reference axis so that when the front wheels turn to the right the left wheels turn to the left. This steering action is achieved when the front wheel steering arms are mechanically or hydraulically connected to move the rear wheel arms in a one to one ratio.
However, there are several reasons why this ideal four wheel steering system is not commercially feasible. The first is the response to steering in a one to one four wheel steering system is so great that the average driver would have difficulty controlling the very rapid steering and would tend to over-steer the vehicle. This is the primary reason why four wheel drive technology which, in its basic form, has not as yet been significantly applied commercially to passenger vehicle where drivers have only average driving ability. Automobile manufacturers have been retiscent to adopt four wheel steering systems because of the large cost required to educate the public to a more responsive steering system.
The second reason for the historical delay in commercially implementing four wheel steering is the tendency of known four wheel steering systems to throw the rear end of the vehicle outside the path of travel of the front wheels during the initial portion of a turning maneuver. At slow speeds this inherent action in known four wheel steerings can cause the rear end of the vehicle to move outwardly a sufficient distance so that it may impact against an object that the front end of the vehicle easily passes, and at high speeds this inherent characteristic can cause the vehicle to go into a spin with a sufficiently abrupt maneuver.
For these reasons there have been many attempts over the last decade to modify the response to the rear wheel steering mechanism so that it is somewhat less responsive than the front wheels to mnimize the rear throw-out characteristic of four wheel steering and to reduce the steering response so that it is more palatable to the average driver.
One of these known response modifying systems has been developed by the Nissan Motor Corporation of Japan, and this system utilizes a control that receives angle information relating to the angular position of the manual steering wheel and applies a first formula to calculate the front wheel angles .theta..sub.F and a second formula to calculate the rear wheel angles .theta..sub.R, with the front wheel angles being inversely proportional to vehicle speed, and the rear wheel angles .theta..sub.R being proportional to the front wheel angles divided by vehicle speed. Thus, the rear wheel angles .theta..sub.R are inversely proportional to vehicle velocity squared V.sup.2 rendering them extremely speed responsive. That is, at low speed this system would create a somewhat responsive four wheel steering system, but at higher speeds would create a very unresponsive system since its responsiveness decreases with the square of the vehicle speed. In any event, the Nissan algorithm for computing rear wheel angle information is constant for a given velocity, and thus operates in the same manner throughout any single turning maneuver for a given vehicle speed.
The problem with the prior Nissan system, as well as most other systems of which applicant is aware, is that they achieve response control merely by suppressing the rear wheel angles .theta..sub.R1, .theta..sub.F2 --.theta..sub.FN by a fixed constant throughout the turning maneuver so long as the variable conditions remain fixed for that turn, i.e., the variable conditions may be vehicle velocity, angular acceleration, surface conditions, etc. This prior approach is a painkiller without a cure.
There have been other recent attempts to provide microprocessors for controlling front wheel angle .theta..sub.F and rear wheel angle .theta..sub.R including a prototype system displayed by General Motors Corporation at the 1987 Automobile Show at McCormick Place in Chicago, Ill., U.S.A. While all details of this system are not publicly available, the system appears to involve algorithmic rear wheel .theta..sub.R suppression with the degree of suppression constant so long as external conditions remain constant.
Most of these prior four wheel steering systems have been the subject of patent applications and resulting issued patents, and there have been a plurality of other systems that are the subject of patent applications and resulting issued patents that have not as yet been commercialized, although applicant is not completely certain which of the following patents fall into that latter category.
The J. I. Case Co. U.S. Pat. No. 4,373,603 shows a four wheel steering system typical of American technology relating to four wheel steering systems particularly designed for construction or agricultural equipment but not really suited to passenger vehicles. In this system three modes of steering are provided, the first being front wheel only, the second being four wheel steering with opposite front and rear direction control, and a third referred to colloquially as "crab" steering where the front and rear wheels turn in the same direction for lateral movement of the longitudinal axis of the vehicle. The three modes are selected manually and there is no sophisticated control system that interrelates these three modes.
The Honda Kaisha U.S. Pat. No. 4,412,594 illustrates an independently computer controlled system for varying rear wheel steering ratio relative to front wheel steering ratio utilizing two steering modes, the first a low speed four wheel conventional mode and a high speed low sensitivity "crab" mode where rear wheel steering ratio suppression is achieved in response to among other factors the angular velocity of the vehicle itself.
Nissan Motor Company has issued U.S. Pat. Nos. 4,418,780 and 4,441,572 that disclose independently controlled front and rear wheel steering that includes a similar two mode high speed crab system where rear wheel steering ratio suppression is responsive to vehicle yawing rate, vehicle spring mass and velocity.
Laurich-Trost U.S. Pat. No. 4,446,941 shows a two mode four wheel steering system in which the first mode includes a mechnical interconnection between the front steering mechanism and the rear, and a second mode includes an electronic rear wheel steering control.
The Honda Kaisha U.S. Pat. No. 4,467,885 shows a four wheel steering system in which there is a mechanical interconnection between the steering system in the front and the steering in the rear with an amplifier-reducer mechanical system for changing the steering ratio of the front vis-a-vis the rear.
The Mazda Motor Corporation U.S. Pat. No. 4,552,239 shows an all mechanical front wheel and rear wheel steering control system in which the rear wheel steering ratio increases with respect to the front ratio in a low speed range and remains substantially constant with changes in the front wheel steering ratio in a high speed range.
The Honda Kaisha U.S. Pat. No. 4,558,877 discloses a spring biased cam arrangement for controlling the steering torque applied to the rear wheels in a four wheel steering system.
The Honda Kaisha U.S. Pat. No. 4,566,710 shows a system similar to the above described Honda system in U.S. Pat. No. 4,467,885 but includes a control wherein the rear wheel ratio changes are effected only when the vehicle is travelling in a straight ahead direction.
The Honda Kaisha U.S. Pat. No. 4,597,462 shows an independently controlled rear wheel steering system for four wheel steering in which the rear wheel steering ratio is modified in accordance with side winds applied to the vehicle.
The Honda Kaisha U.S. Pat. No. 4,598,788 discloses a four wheel steering system in which the steering ratio of the rear wheels is controlled in accordance with wheel cushioning of the various axles.
The Mazda Motor Corporation U.S. Pat. No. 4,601,357 shows a four wheel system similar to the Honda U.S. Pat. No. 4,566,710 system, but here rear wheel steering ratio control is inhibited during a turning maneuver.
The Mazda Motor Corporation U.S. Pat. No. 4,621,702 discloses an all mechanical steering system for four wheel steering that includes a differential positioned between the front and rear wheel steering systems for varying the steering ratio of the rear wheels.
The Nissan Motor Company U.S. Pat. No. 4,625,822 discloses a four wheel steering system for a non-articulated vehicle with a high speed crab control mode where the rear wheel angles are increased slowly to a target value at the initiation of a turn to prevent the rear end of the vehicle from swinging laterally outwardly.
The Kiki Company U.S. Pat. No. 4,645,025 discloses a computer controlled system for the rear wheel steering mechanism in a four wheel steering vehicle where a memory permanently stores a program of rear wheel angle information which is picked off in response to the angles of the vehicle as they turn.
It is a primary object of the present invention to ameliorate the problems described above in four wheel steering systems for non-articulated vehicles.